Miss Blackman's father, Tim Blackman, and her sister Sophie, were in court to hear the verdict. They immediately left to consult their lawyers and have not yet commented on Obara's acquittal.

BBC Tokyo correspondent Chris Hogg said that under Japanese law the Blackman family themselves have no grounds to appeal against the court's decision.

They could try to mount a form of civil action but as Obara has been declared bankrupt there would seem to be little point.

Our correspondent said Obara showed little emotion as the judge told him he would be sentenced to life in prison for killing Carita Ridgway in 1992 and eight other rapes.

Obara lured Miss Ridgway to his apartment just south of Tokyo, where he drugged her and raped her. She later died in hospital of liver failure.

Videotapes found

A video seized from his home showed him attacking Miss Ridgway and using a towel soaked with chloroform to keep her unconscious.

Samantha Termini holds a picture of her sister, Carita Ridgway

Miss Ridgway's mother, Annette Foster, welcomed the conviction but criticised the Japanese police.

"If Obara had been investigated in 1992, it would have stopped the crimes he committed for the next eight years," she said.

Obara admitted he had been with Miss Blackman on the day she disappeared but claimed she became unwell at his apartment after taking drugs.

He said he then called an acquaintance, known by the nickname Kacchan, and asked him to take her back to Tokyo. Kacchan has since died which meant his story could not be challenged.

Judge Tsutomu Tochigi said: "There is nothing to prove that [Obara] was involved in the rape and her death. The court cannot prove he was single-handedly involved in her death."

The judge said it was clear the victim and the accused were together before she vanished and then died but he said this was not enough to secure a conviction.

Planning to appeal

Obara's lawyer, Yasuo Shionoya, said his client would lodge an appeal against his conviction in the Carita Ridgway case.

"Regarding Carita's case, I think we will have to file an appeal. It's very plausible that we would file an appeal in the other cases as well," said Mr Shionoya.

The judge said Ms Ridgway, who died of hepatitis at a Tokyo hospital in February 1992, had been drugged with chloroform by Obara.

Joji Obara has been given a life sentence

But Mr Shionoya said: "I doubt whether liver failure could have been triggered by the use of chloroform."

Obara was arrested on charges of rape resulting in the death of Ms Blackman in 2001 and had been on trial at Tokyo District Court since 2003.

Mr Blackman, from the Isle of Wight, spent thousands of pounds travelling to Japan to try to get the local police to investigate his daughter's disappearance as a suspected crime.

Eventually, after he enlisted the support of UK Prime Minister Tony Blair and his Japanese counterpart, the police arrested Obara, a property developer who had a seaside home at Miura, outside Tokyo.

Last year Mr Blackman accepted 100 million yen (£450,000) from a friend of Obara, but he denied it was "blood money" and said such "offers of condolence" were common in Japan and did not affect the court case.